Industrial pumping takes many forms, all with the general requirement of transporting fluids or slurries through a process stream. Pumps are selected based on the application requirements including head pressure, metering accuracy, temperature, particle tolerance, fluid viscosity, cost, safety, service rate and a variety of other parameters. Pumps can generally be classified in two categories. Positive displacement pumps isolate discrete volumes of the working fluid and force them to move in a controlled direction. Kinetic pumps operate by adding kinetic energy to the system which creates a local increase in fluid velocity. Kinetic energy is converted to potential energy, i.e. pressure, at the pump output.
Reciprocating pumps, such as pump 2 shown in the diagram of FIG. 1, remain a primary industrial means of pumping fluids when high purity, high pressure (e.g. >100 bar to more than 1000 bar) and high precision (e.g. <1% flow variation) are needed. Reciprocating pumps come in several formats including mechanical and pneumatic piston pumps, and mechanical and hydraulic diaphragm pumps. Such pumps 2 are characterized by having one or more pump heads 4 which transfer fluid between a low pressure input and a higher pressure output. Each pump head 4 contains a means of physically adjusting the internal volume available to the pumped fluid. In operation, each pump head 4 uses a piston 6 driven by a cam 8 that alternately aspirates fluid from the input 10 by increasing the available pump head volume, then dispenses the fluid to the output 12 by decreasing this volume. Most reciprocating pumps are designed to flow in only one direction. Flow direction is controlled by a series of check valves 14, 16 that isolate the pump head from the output pressure during aspiration and from the input pressure during dispensing. The output pressure is generally controlled, not by the pump, but rather by the downstream resistance-to-flow of the process flow stream serviced by the pump.
Reciprocating pumps are characterized by the number of pump heads they utilize. A pump with a single pump head is referred to as a simplex pump. Duplex, Triplex and Quad pumps refer to pumps with two, three and four heads respectively. Two or more pumps heads are required to provide pseudo-continuous flow since one pump head can be delivering while the other is aspirating. However, since the very nature of the movement involves stopping and restarting in opposing motions, reciprocating pumps can only emulate continuous rotary pumps approximately. In general, the greater number of pump heads for a given flow rate, the lower the pulsation of the output stream.
When fluid being pumped by a piston pump is relatively incompressible, these pumps are frequently referred to as metering pumps, since the volumetric flow of the fluid is presumed to match the mechanical volumetric displacement of the piston or diaphragm in the pump head. An example of a metering application of a reciprocating pump is a low pressure syringe pump, in which a glass syringe draws in an aqueous solution and dispenses it very accurately to a downstream reservoir. Under this low pressure use (generally less than 2 bar) the volumetric compression of aqueous solutions is almost immeasurable and thus the presumption of accurate displacement is correct.
When reciprocating pumps are used with very compressible fluids such as permanent gasses, they are frequently called compressors or gas boosters. Gas boosters represent an ideal example of the influence of fluid compressibility on pump performance. In this case, the typical application is to increase the pressure of the gas between the input and output. A fundamental characteristic of gas boosters is the compression ratio. The compression ratio is simply the ratio of the maximum fluid volume a pump head can isolate between its check valves at the peak of its intake stroke to the minimum volume it can reduce to at the end of its delivery stroke. Hence, a compression ratio of 7:1 indicates the total volume at intake is seven times greater than the residual fluid volume at the end of delivery.
FIG. 2 shows the basic components for an HPLC pump of prior art. HPLC pump 18 is an example of an electric cam driven pump. In this case motor 20 rotates shaft 22 to rotate eccentric cams 24 and 26 to provide a reciprocating motion of pistons 28 and 30 contained in pump heads 32 and 34 respectively. As each piston aspirates, fluid is drawn from fluid reservoir 36 through input check valve 38 or 40 respectively. Output check valve 42 or 44 remains sealed during aspiration. During the delivery stroke, input check valve 38 or 40 is shut while output check valve 42 or 44 opens to deliver fluid to process stream 46. The cam drive shown in FIG. 2 is just one example of an HPLC pump. Others would include ball screw drives, pneumatic drives and hydraulic drives coupled to the pistons 28 and 30. Much of the remaining discussion focuses on pumping a fluid using compression compensation of laboratory-type HPLC and SFC type pumps that are similar in design to HPLC pump 18.
Requirements for pumps used in typical laboratory HPLC instruments are very demanding. Pumps must be able to deliver at very high pressures (up to 400 bar for traditional HPLC and as high as 1000 bar for recent ultrahigh performance LC systems). A 2000 bar ultrahigh performance LC system is expected. HPLC pumps must also be able to handle fluids of ultra-high purity without contributing detectable contamination. In addition, for a given flow rate, the volumetric delivery of fluid is expected to remain constant within narrow limits (<1% variation) across the majority of the operational pressure range. Finally, the same pump is also expected to vary flow precisely over at least an order of magnitude of range in periods as short as one minute. This is the result of the need for a technique called gradient elution in which the two solvents controlled by separate pumps are systematically adjusted in relative composition from a weakly to a strongly eluting mixture while maintaining a constant combined flow rate.
Normally, an unmodified high performance liquid chromatography (HPLC) pump would deliver an unknown and varying amount of a compressible fluid under such conditions. As the column head pressure increases during the gradient, a larger percentage of each pump stroke would be used up compressing the fluid instead of delivering flow. With an uncompensated pump, the delivery rate becomes a smaller fraction of the flow setpoint. When a second pump is added to a system to deliver an incompressible fluid under high pressure, its delivery rate is unaffected by the increasing pressure. Subsequently the two pumps deliver inaccurate flow and composition to the mobile phase. As the pressure in the system rises, the total flow drops below its setpoint, but the concentration of the modifier increases beyond the modifier setpoint. The temperature of the compressible fluid in the pump head must be controlled to prevent the delivered mass flow from changing even further.
When compressed, a pumping fluid heats up and attempts to expand. For highly compressible carbon dioxide at outlet pump pressures above 200 bar, a temperature rise of more than ten degrees centigrade is possible within the fluid. The rapid compression of the pumping fluid causes the fluid to heat up and expand and the density to decrease. When heat is transferred to the pump body, the pumped fluid cools and the fluid density increases. Compressibility levels encountered in ultrahigh performance chromatographic systems are very similar to those encountered in supercritical fluid chromatography (SFC) over the last twenty years. In the past, almost all pumps sold to perform SFC or supercritical fluid extraction (SFE) were modified versions of high performance liquid chromatography (HPLC) pumps.
SFC is a separation technique closely related to high performance liquid chromatography (HPLC), except that one of the fluids used in the mobile phase is a liquefied gas. The fluids are generally compressed and are used at pressures above 80-100 atmospheres. The most common fluid in SFC is carbon dioxide but other fluids have sometimes been used. The carbon dioxide is stored in a supply cylinder, as a gas, in equilibrium with a liquid phase, under pressure. Depending on “room temperature”, the pressure in the supply cylinder may be approximately 40 to 60 atmospheres. It is much more difficult to pump carbon dioxide than a normal liquid, since its compressibility is very high and it readily expands to a gas at room temperature, unless an external pressure is applied.
SFC uses liquefied CO2 as one of the components of the mobile phase. As a liquefied gas, CO2 must be delivered at high pressure to the pump head in order to remain in the liquid state. This is normally accomplished by connecting a tank containing both liquid and vapor CO2 in thermal equilibrium. A dip tube in communication with the CO2 liquid of the tank is plumbed directly to the pump head, however, the CO2 may become partially or fully vaporized prior to entering the pump head. Thus, pre-conditioning of the CO2 flowstream and pump head becomes essential to proper operation of the SFC or HPLC system. Generally, chilling of the pump head and pre-chilling of the fluid are both necessary to insure that CO2 remains in the liquid state during pump aspiration. Special grades of high purity CO2 are used in SFC to prevent dissolved components of less pure CO2 from affecting the optical clarity of the mobile phase. Mixtures of CO2 and common organic solvents also tend to have higher changes in refractive index than corresponding water: organic solvent mixtures so that small rapid variations in composition are more observable with optical detectors.
When the pump attempts to fill, the pressure in the tube may drop below the pressure in the supply cylinder and some of the fluid may vaporize. The motor on the pump and the associated electronics tend to warm the pump, to above room temperature (i.e., 30-35° C.). If the pump temperature is greater than the supply cylinder temperature, the fluid will further vaporize inside the pump head. Since modern HPLC pumps have relatively low compression ratios they cannot function effectively as gas compressors. The fluid must be present in the pump as a liquid. Modified HPLC pumps must perform both compression and metering of the carbon dioxide to produce accurate flows. HPLC pumps do not normally need to be chilled. However, an SFC pump must be cold enough to not vaporize liquid carbon dioxide entering the pump head (i.e., it must be colder than the supply cylinder. In order to insure the pump head is cold enough to pump liquid carbon dioxide, it is generally cooled to significantly below ambient temperature. Pre-chilling the fluid, before it enters the pump lowers its compressibility.
As mentioned, pumping of liquid CO2 takes special precautions to insure a continuous liquid supply into the pump head. The compressibility of liquid CO2 is also a major factor since it is typically as much as ten fold higher than most of the organic liquids. Further, compression of CO2 between 60 bar (approximate tank pressure) and 400 bar (the maximum system pressure) can raise the fluid temperature more than 25° C. Such a temperature rise dramatically alters the density of the delivered fluid and introduces even more requirements for pump control.
In a pump performing both compression and metering, the actual mass flow delivered by the pump depends on the pump head temperature. To insure accurate and repeatable flow rate, the pump head and carbon dioxide temperature must be both below the temperature of the supply cylinder, and tightly controlled. To perform SFC with pre-existing HPLC pumps, chillers were connected to the pump heads. Hewlett Packard introduced the first commercial SFC in 1982 using a modified 1084 HPLC. The HPLC pumps used were massive, with large electric motors. It was necessary to cool the pump heads to keep the carbon dioxide liquid, and prevent cavitation. The SFC pumps employed a heat exchanger consisting of hollow compartments made of brass, bolted to each pump head. An external, commercial circulating bath pumped a mixture of water/antifreeze through tubing connected to the heat exchangers. The temperature of the circulating bath was set to −20° C. The chilled liquid was circulated through the chambers bolted to the HPLC pump head, by the circulating pump in the bath, chilling the pump heads. In addition, a coil of tubing was placed inside the circulating bath allowing the fluid to be pre-chilled before entering the pump. In spite of all these efforts, the pumps sometimes cavitated, which resulted in inaccurate flow.
HPLC pumps were not intended to be used with vapor phase carbon dioxide. They were used with supply cylinders with “dip” tubes, a tube that extended from the cylinder valve to near the bottom of the cylinder. The liquid layer of carbon dioxide was withdrawn from the cylinder, chilled, and presented to the pumps. Use of the liquid layer from the cylinder eases pumping problems, and decreases the cooling load on the chiller, but exposes the samples to potential contamination in the carbon dioxide. Liquid carbon dioxide can act as a solvent for many relatively non-polar compounds including greases. To avoid contamination, a special grade of carbon dioxide was developed (SFC Grade) that was guaranteed to be pure. This SFC Grade carbon dioxide cost up to 15 times more than many common industrial grades.
Since, in such SFC units, the chiller, the tubing of the circulating bath, the connecting tubing delivering the fluid to the pump head, the heat exchanger on the pump head and the pump head itself, were all below the freezing point of water, humidity in the laboratory air condensed and/or froze on the pump head and tubes. The use of sub-0° C. pump head settings sometimes resulted in large blocks of ice forming on the CO2 pump from ambient humidity in the lab air. This condensation and freezing has no relation to the intended purpose of the chiller and is wasted energy, which forces the use of a much larger circulating chiller than is actually necessary to chill the fluid and pump-head. The size of the external chiller required depends on the relative humidity of the lab air, which can vary greatly throughout a year. Subsequently, the chiller must be sized for the worst case conditions, which is far larger than nominal. Further, some prior solutions to remove the ice formations include isolating pump heads with insulation and blowing hot air onto the iced pump head, which creates even more wasted energy in the ironic situation of air heating of a pump head area that is at the same time being chilled to remove internal heat. Collection and safe removal of the condensate creates an additional undesirable complication. The CO2 delivery pumps are generally fitted with a leak sensor that shuts off the power if a leak (or condensation) is detected. The condensation, although not technically a leak, can trigger the leak sensor.
The ice and condensation problems inspired later SFC pump designers to use pump head temperatures slightly above freezing, such as 5° C. For example, Thar/Waters corporation designers currently use an external chiller connected to the pump heads with large bore tubes, very similar to the design of the original 1084. The Jasco SFC/SFE (Supercritical Extraction System) initially used this approach when introduced in 1985. The Gilson system, introduced in 1992, used this approach. However, operating above freezing only partially solves the problems related to SFC pumps. A chiller mounted on or anywhere around a pump head will still have cold surfaces exposed to humid ambient air, resulting in copious amounts of condensate that robs the chiller of most of its power while creating an unwanted waste stream. The chiller must still be sized to be much larger than actually needed to chill the carbon dioxide and pump head.
Using a separate commercial circulating bath to chill an HPLC pump can make transport and servicing of the individual components difficult and messy. In typical systems, a commercial circulating bath, with dimensions of 2.5 to 5 cubic feet is used, which is connected to a separate pump module with 4-6 feet of ⅜ths to ½ inch OD tubing. Some chillers provide 2500 Watts of cooling (or more). Such circulating baths have their own control electronics and power cable, are bulky, and are expensive. Since previous embodiments chilled the circulating fluid to below room temperature, the connecting tubing had to be heavily insulated, making it extremely thick, and unwieldy, yet excessive condensation on the tubing is still common.
The only other common chiller used in SFC employed Peltier thermoelectric elements directly on the pump head along with finned heat exchangers cooled by forced convection of lab air. With the passage of electric current, one side of the Peltier device becomes cold, directly chilling the pump head, while the other side becomes hot. A fan blows room temperature air through the finned heat exchanger, mounted on the hot side, removing the heat from the heat exchanger. This arrangement eliminates the need for an external chiller, connected to the pump with large bore elastomeric tubing, filled with cold, circulated fluid.
The first SFC pump to use this Peltier configuration was introduced by Hewlett Packard in 1992. In one prior design (Haertle, energy efficient pump)), a heat exchanger was embedded in the pump head to pre-chill the liquid carbon dioxide before it entered the pump, while the Peltier device lowered the pump head temperature to 4-5° C. In yet another embodiment by Haertle, there was a counter flow heat exchanger used that contacted the incoming carbon dioxide with the carbon dioxide exiting the pump to pre-chill the fluid and attempt to minimize the amount of cooling power needed. The finned heat exchanger and a high volume fan were mounted directly on top of the pump head. The need to have large volumes of air contact the heat exchanger on one face of the Peltier device made it difficult to isolate the other cold face of the Peltier device from the large volumes of moist air, since they were only separated by ≈⅛th inch. Water vapor from the laboratory air condensed on the pump head, robbing it of efficiency. The Peltier elements had to be over-sized to provide many times the cooling power actually needed to pump CO2, in order to cope with the unwanted condensation. Extensive insulation tended to minimize this condensation, but blocked easy access to the pump head for routine maintenance.
Another significant problem is how to maintain a pump that has problems with chiller designs described above. All pumps require routine maintenance, such as periodic replacement of check valves and the main seals on pistons. Typically, a large finned heat sink is mounted on the front of the pump which completely blocks access to the pump head for routine maintenance. The need to remove the chiller for access, complicates maintenance. Jasco, Selerity, and SSI all pursued similar approaches with the heat exchanger mounted directly on the pump head, but left out the heat exchangers in the Haertle patents. Thar/Waters has announced a similar approach in upcoming products. All these approaches require excessively large Peltier devices or elements, and complicate routine maintenance.
A further problem is that although often powerful, all previous condensers used in SFC could not guarantee that the fluid in the pump was actually a liquid or that the mass flow delivered was accurate.